Variable delay means



Aug. 9, 1949. B. M. OLIVER VARIABLE DELAY MEANS -7 Sheets-Sheet l Filed Aug. l, 1944 /A/vfA/TOR B. M. OL/ VER U M m l A Aug. 9., 1949. 5. M. OLIVER VARIABLE DELAY MEANS 7 sheets-sheet 2 Filed Aug. l, 1944 TNLbO IIQQMF M E n am R. r @n N NL J M0 B. V, B a,

Aw"- 9 3949. B. M. oLlvER VARIABLE DELAY MEANS Filed Aug. 1, 1944 7 sheets-Sheet I 5 /NI/ENTOR v 5M OLIVER S. ATTORNEY oo 7 7, o0 7 4, 2

Aug. 9, 1949.

B. M. OLIVER VARIABLE DELAY MEANS l 7 Sheets-Sheet 4 Filed Aug. l, 1944 /Nl/ENTOR B. M. OL VER J. /Z/b ATTORNEY Aug. 9, 1949. B. M. OLIVER 2,478,778

VARIABLE DELAY MEANS Filed Aug. l, 1944 '7 Sheets-Sheet 5 l F/G. 5.

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' VARIABLE DELAY MEANS Filed Aug. 1, 1944 'I sheets-sheet 7 T/ME /N sEco/vos l /N I/E/V TOR M 0L VER T TORNEV Patented Aug. 9, 1949 UNITED STATES PATENTToFF-lcs VARIABLE DELAY MEANS Bernard M. Oliver, New York, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application August 1, 1944, Serial No. 547,573

Claims.

This invention relates to electric transmission and more speciiically to delay means for electric waves which is suitable for delaying pulse trains. This application is a continuation-in-part of application Serial No. 491,829, filed June 22, 1943.

It is an object of this invention to provide novel electric circuits adapted to variably transmit pulses or other wave forms.

It is another object of this invention to provide a novel variable transmission line adapted forl rapid adjustment.

It is still another object of this invention to provide a novel variable delay line or unit which is suitable for use in radio object locating and distance measuring systems.

It is a further object of this invention to provide a novel transmission line having a variable delay but which has a substantially iiat attenuation versus delay characteristic.

While the delay line or unit of this invention will be shown and described in connection with a radio object locating and distance measuring system (frequently called a radar system) of the automatic range tracking type, it will be apparent from the description that this line or unit can be used wherever it is desired to delay a wave by dilierent amounts at different times.

In accordance with an illustrative embodiment of the invention, there is providedA a variable delay transmission line comprising a number of sections of inductance and capacity, the inductance elements being in one conductor of the line and the capacity elements across the two conductors of the line. All of the inductance elements are of equal inductance and the capacity elements all have the same capacity, there being single sections connected between adjacent ones of twelve contact points of a rst or input rotary switch member and twelve sections connected between adjacent ones of the contact points of a second or output rotary switch member. The moving contact arm of the first switch member is actuated in either direction by a servo-motor to cause the arm to snap from one contact or tap to the next and the moving arm of the second switchmember, which is similar to the first, is geared to the arm of the rst switch member by counter-gear mechanism so as to snap the second arm one tap when the rst arm snaps from its last tap to its rst or vice versa. The delay introduced by the line is proportional to the number of sections between the input and output taps. The output taps are brought out of capacity voltage dividers each comprising two condensers in series replacing at the tap point, and having the same over-all capacity as, the single shunt condenser which would otherwise be present. The output is taken from across the condenser in each output pair farthest removed from the line conductor which contains the inductance elements. The capacity members of each divider are made of such size thatthe signal in the output path has the same magnitude as in all other output paths. It follows that the ratio of thecapacity of the condenser across which the output voltage is taken to the capacity of the two members in series decreases as the output tap number increases by an amount to make the amplitude of the output signal the same regardless of the position from which it is taken. The same voltage-dividing principle may be applied to the input taps but, in the usual practical case, the. attenuation between the input taps is not great enough to warrant this renement. Adjacent inductance members (one in each section) are wound to have mutual inductance between them. By properly adjusting the coecient of coupling, a flatter delay characteristic can be obtained than with no mutual inductance, i. e., the delay can be made constant (for a particular setting) to higher frequencies.

If each selected echo pulse (the one corresponding to the target being tracked) is not symmetrically positioned'in time with respect to theV corresponding two gating pulses, a differential current is produced which is utilized to drive a motor to vary theA position of the switch arms of the variable delay transmission line to change thereby the time displacement between the gating pulses and the corresponding transmitted pulses. The motor operates until the differential current is zero, for which condition the pairs of gating pulses are symmetrically positioned with lrespecttothe corresponding selected echo pulses. The dial indicator on the variable delay transmission line then indicates the range of the selected target and continues to indicate accurately the range even though there is relative movement between the target and the observing station. Y

` The invention will be more readily understood by referring to the following description taken in connection with the accompanying drawings forming a part thereof, in which:

Fig. 1 is a schematic block diagram of an object detecting and distance measuring system of the pulse reflection type employing a variable delay line in accordance with the invention;

Figs. 2, 3 and 4 are detailed circuit diagrams of portionsof the systemV schematically shown in Fig. 1, the variable delay line of this invention being shown in Fig. 2; and

Figs. 5 to '7, inclusive, are various schematic and graphical representations to aid in understanding' the invention. u i Y Referring more specifically tov the. drawings, Fig. 1 shows, by way of example to illustrate the invention and in block diagram form, a Variable delay line used in an object detecting and distance measuring sytem of the pulse reection type which is adapted to automatically track (in range) a desired target by continuously indicating the range of the target from the observer-s station on a dial or dials to provide a mechanical motion representative of the distance between the target and the observing station. Briefly stated, the system comprises means for radiating pulses which are aimed in a desired direction by means of a proper antenna system, means for receiving the original pulses and also reflections (choes) from one or more objects or targets, means for producing by the variable delay line of this invention a delayed pulse a variable period of time after the formation of each transmitted pulse, and apparatus responsive jointly to the delayed pulses and the received signals (including the echo pulses corresponding to' a target it is desired to track) to vary the delay period of time so that a predetermined relatlonship is maintained between the time of occurrence of a reflected (echo) pulse representative of the selected target and the delayed pulse corresponding thereto. The length of the delay interval is utilized to indicate the range of the target and if, as in this invention, the means for producing the delay is continuously operable, properly calibrated readings of the delay interval give the range of the target at all times.

In the arrangement of Fig. 1, a transmitter I is provided to supply ultra-high frequency radio waves which can be directed at a particular object by means of an antenna II. Any suitable transmitter and antenna can be provided. For example, the transmitter I0V can comprise an oscillator I2 for providing a sine wave having a suitable periodicity which can conveniently be 400 cycles per second, for example. This oscillator energizes a pulse generator I3 of any of several suitble types well known in the art. For example, see United States Patent 2,117,752, issued May 17, 11938, to L. R. Wrathall which provides an energy pulse at a particular point of each cycle of the input wave provided to it. The pulse from the member I3 is then applied to a carrier generator and modulator I4 of any suitable type. The member I4 may, forl example, generate a carrierV wave of approximately 3300 megacycles and this carrier wave is modulated by the pulses from the pulse generator I3 and the modulated wave applied to the antenna II. Unmodulated pulses from the Ygenerator I3 are also applied by means of the connection I5 to the amplifier I6. The purpose of this connection will be explained more fully below.

Waves reflected from one or more objects within the range of the transmitting antenna II are received by a receiving antenna I'I. While the an'- tennas II and I'I can be of any suitable type'. preferably they are of the polystyrene polyrod type disclosed in an application of G. E. Mueller, Serial No. 469,284, filed December 17, 1942, and which issued as Patent No. 2,425,336 on August l2, 1947. Connections to the transmitting antenna II and from the receiving antenna II are made by coaxial cable. The reii'ected waves picked up by the receiving antenna II (and aiso the transmitted waves) are applied to a receiver I8 of any suitable form wherein it is amplified, rectified and fed into two amplifiers 28 and 29 the input circuits of which are in parallel. If desired, a fixed delay circuit 43 can be included in the circuit between the receiver I8 and the amplifiers 28 and 29, the purpose of which will be described more fully below.

At this point it is advisable to leave the received signal temporarily and describe the formation of timing pulses which are generated at the same periodicity as the transmitted' pulses but are delayedby a variable period of time from the corresponding transmitted pulses. Reference will also be made to Fig. 2 which is a more detailed drawing of an amplifier I6 for pulses from the pulse' generator I3, a low-pass filter 2l connected to the amplifier I6, a linear amplifier 22 following the low-pass filter, and a variable delay line 23 connected to the linear amplifier. The so-called timing pulses appear in the output of the variable deiay line.

Pulses from the pulse generator I3 (which ordinarily are in phase with the transmitted pulses but which may lead or. lag them, if desired) are applied to the input circuit of the amplifier I6 which is shown as a single tube VI. The tube VI includes an anode 30, a cathode 3l, a control grid 32, a screen grid 33, and a suppressor grid 34. The circuit between the control grid and the cathode 3`I includes a portion ofa potentiometer resistor 35 connected across thev input terminals and an` anti-sing resistor 36 (to prevent parasitic oscillations) connected between an inner terminal of the resistor 35 and the control grid 32. The cathode 3l and the suppressor grid 34 are connected to ground in accordance with wellknown practice. The plate 30 is connected through an anode vresistor 3l to the positive terminal of the power supply 38 (represented schematically by a box), the negative terminal of which is grounded. The screen grid 33 is connected through the resistor 39 to the positive terminal of the source 38, the end of the resistor 39 remote from the screenV gridy being connected through a ,by-pass condenser 40 to ground. If the pulsesapplied to the input circuit of the tube VI fromithe pulsegenerator I3 are negative, there appears. in the output circuit of the tube VI a succession of positive pulses. The anode 30 of thetube-VI. is connected` through a coupling condenser 4I to'one terminal of a low-pass filter (the other terminal being` connected to'ground) which is used as an interstage coupling between the tube VI and the tube V2 to prevent any component above 2 megacycles, for example, from being applied to the grid of the tube V2 which is a linear amplifier for the positive pulses. V2 is a linearV amplifier, its output will contain no new frequencies, and the input to the delay line will contain only frequencies below the cut-olf of the low-'pass filter. If this cut-off is chosen low enough so that the delay line has negligible delay distortion over the' pass-band, there will be no asymmetrical' distortion of the pulse by the Since lines. The low-passl lter .2| can be f any suitable form. y

The tube V2 includes an. anode. 50, a cathode 5|, a control grid 52, a screen grid 53, and a suppressor grid 54. The circuit between the control grid 52 and the cathode 5| includes an anti-sing resistor 55;, a leak resistor 56 connected between the output terminals of the filter 2|, and the parallelconnected resistor 51 and condenser 58 which'are connected both in the input and in the output circuits of thetube V2 to provide bias for the tube. Resistors v31 and 56 also serve to terminate the lter 2l. The anode 50 is connected through the usual anode .resistor 59 to the positive terminal of the source 38 while the screen grid 53 is connected to this terminal through the anti-sing resistor 60. A by-pass condenser 6| is provided between the outer terminal of the resistor 68 and ground B9.

The outputnegative pulses of the tube V2 are fed to the variable delay line 23 through a coupling condenser 62. The delay line 23 is designed to be substantially flat in gain and -of constant delay (linear in phase) upto about 2 megacycles and is terminated at each end by a resistor 10. It

comprises a number of sections of inductance and capacity, there being single sections between adjacent ones of the contact points 1|, 12 8|, 82 of the input snap switch member (switch No. l

in the drawing) and there being twelve sections between adjacent ones of the contact points 85, 88 95, 96 of the output snap switch member (switch No. 2 in the drawing). Moving contact member 83 which, as will be described more fully below, is actuated in either direction by a servomotor, is adapted to contact members 1I' to 82, inclusive, while moving contact member 84 is adapted to contact the members 85- to 96, inclusive. The contact members 1| to 82, inclusive, are preferably arranged in a closedV circle (asin the drawing) in such a way that contact member 83 if it is going in that direction) contacts-successively 1|, 12 8|, 82 and then 1| again. The member 83 can also contact these elements in the reverse direction. When rotary contact member 83 moves in a clockwise direction from the contact element 82 to the contact 1 by means of any suitable counter gear mechanism 91, the rotary switch member 84 is adapted to move .one tap (corresponding yto twelve sections like that between 1| and 12, 12 and 13, etc.) in a clockwise direction. The switch members 83 and 84 preferably employ micro-switch blades having snap action between contacts.

8i', 82 can be arranged in a line and the contact member 83 moved in a linear path, the design being such that when the contact member 83 is moving towards the left and passes the contact element 82 (the last one), a suitablereturnY mechanism returns'the member 83 to the contact element 1| and at the same time snaps the contact member 84 one tap to the right. The same thing applies, but in the opposite sense, vwhen the contact memberl 33 is moving toward the right. As in the case where the contact elements are arranged in circles, the rotary contact members 83 and Mare actuated in either direction by a servo- Vmotor which is controlled in a manner which will be described hereinafter.A Inthe position shown in the drawing, the pulses from the tube V2 are Adelayed by a period'corresponding to the length .of time vit takes a pulseto traversethe delay sec- Any suitable gearing B1, such as that in the well-known Veeder counting. mechanisms, may be utilized for this purpose. As Van alternative, the contact elements 1|, 12

tionsbetweencontact elements Bland 1| (thatfsi, between the terminals of the transmission-line marked IN|0 and'INfU), and also the delay sections between contact elements 85 and 86 (that is, between the'terminalsvof the transmission line marked OUT-8 and OUT-I The input pulses propagate both ways down the line from the point of injection (point IN-l if switch element 83 is in the position shown in the drawing) and are absorbed at either end by the resistors 10. The output is to a high impedance load which ,merely samples the voltage of the passing pulse. The delay introduced by the line is accordingly proportional to the number of sections between the input and output taps.Y While a number of equally spaced taps can be utilized, the arrangement shownin Fig. 2 is preferred as by means ofV this arrangement` the number of leads is minimized. rThe input taps, twelve in number (IN-8 to IN-l connected respectively to switch elements 1| to 82, inclusive) are separated by onel section each and the output taps (OUT- 8 to OUT- ll 'connected respectivelyV to switch elements 85 to 98, inclusive) are, as pointed out above, each separated byrtwelve sections.- The rotary-switches are, of course, reversibleA and can be rotated continuously in either direction. In this way'delays each of one section per step, are obtained, there being 144 steps but requiring only 24 leads. Each section includes. an inductance member and a capacity member,v adjacent inductancemembers having mutual inductance between them. This mutual inductance may be obtained by winding all of the inductances series aiding ona long form and is then in such a sense as to. introduce an equivalent negative inductance in series with the shunt condenser between the coils. Byproperly adjusting the coefficient of coupling a flatter delay characteristic can be obtained than with no mutual inductance, i. e., the delay can be made constant to higherfrequencies.

To compensate for the changing attenuation introduced by the line as the number of intervening sections is changed, the output taps are brought out of capacity voltage ,dividers |80,

lill .,I i8, each of. two capacity members, the capacity members of each divider being of -such size' that their attenuation plus that ofthe line up to them Vis constant. In other words, the ratio of the capacity of the lower capacity member (from which the output voltage is taken). in each divider to that of the overall capacity ofthe two members in that divider decreases as the output tap number increases by an amount to make the amplitude of the output signal the same regardless of the position from which it is taken. i The output pulse is therefore merely delayed without having any appreciable variable change in shape. The attenuation between the input taps is usually not great enough to warrant the provision of capa-city dividers of this type.

yThe delayed pulse (called also the marking or "timing pulse and shown in Fig. 6A) at the output terminal (connectedto the contact member 84,) is applied to a first gate-forming circuit-24 comprising (see Fig. 3) the tubes V3, V4, V5 and V5. The tube V3 acts as'an amplier and includes an anode |28, aV cathode 12|, a

Acontrol grid |22; a screen grid |23,` and a, sup- Y pressor grid |24. The control grid l|22 is, con nected through an anti-sing resistor |25 and a grid leak |26 to ground, this-being vthe potential ofthe cathode |2| and of the suppressorV grid |24. The anode is connected through resistor .|21,'inductance member |28' (to provide high'frecarefree quency equalization); .and :resistor :|29 `to :the terminal of the direct current source `|30 orftothe source 38, if'desred), While-the tscreen grid is connected through .the anti-sing resistor and the resistor |129 toitheisanoe teriminal. The'end of the resistor |3| :remote vfrom .the screen igrid'is .connected through `the by-pass condenser :|32 to ground.

`The `anode `of the tube V3 is connected ito'ithe ntrolgridlh ofthe tube V4 -throughracoupling Acondenser `|33 Sand an anti-sing'resistor 1374. Ihe fgrid lvoltage of this tube Ve `is shown v'in Fig. 6B. The terminal of the resistor1|34 re# mote :from -the control grid is connected .to

lground :throughthefgrid vleak resistor 135. The j Stube vdialsoincludes-an anode |40,*a.c'athod'e |41, ascreenrgridMS and asuppressor `gridMli. The suppressor grid '|44 and the cathode |42 are conjrfected `to ground through the parallel-connected r'esi'stancemember V| 45 and capacity 'member I 46 whichserve to r.provide bias Vfor the control grid. The"bleeder resistor' |41 i-s also connected be r.tweenthe :cathodesand the positive terminal Kof thefsource |30 through the resistor |48 to `place th'ecathode '|41 ata positive .potentials even when no currentiis flowing `through the ftube. In fthe iabsence'of signal pulses, the tube V4 is .adapted fto ibecut oi. The suppressor grid |43 is confnected'throughithe anti-sing resistor |36 and `th'eire'slstor |48 to the positive terminal of the `:source |30 Y'While the 4anode |40 is connected throughthe :resistor Maand the resistor |48 to thesame'terminal of fthe'fsource |30. A by-pass fc'ondenser il 50 is' provided between'the screen .grid |13 and ground,

The 4anode |40 -is connected through a1con plingrcondenser v15| "to the control grid |52 ofthe tube V5,this tube also including an vanode |60, `a *cathode 4|'6|, 'a'screen `grid |63, anda suppressor 4grid 164. A tuned interstage circuit comprising the condenser |52 and'inductance member `|l53 'is Econnected :in parallel -between'the control grid -|f621and ground. One or more of themembers' |52 :and |53 may `be ivariable, if desired. -The interdstage coupling, which is designated the ytuned circu'it Ti, is shook-excited by pulses n from th'e `tube V4 but the oscillation initiated' lby I the shock excitation is "damped within -one-fourth `of an iosc'ill'ation .cycle fby means of thetube V5 which startsite .draw gridcurren't when the oscillation Spoteritialpasses the cut-oibias point of tube' V5. 'Ihec'althode` I0 fand'the suppressor grid 54 are fioon'necte'd to ground. '.ThcLscreen A'grid '|63 is connected through an fanti-sing resistor |65 andresistors |66 and |01'to`thefpositive terminal foflthe source |30. The terminal ofthe resistor 3.65 'remote ."from the Lscreenl grid is oonnectedvto `ground'through the f condenser "|12, while'the plate 1'I5'0`isvvconnected Vthrough 'the resistor |68 andthe :resistor |61 to the positive-terminal y'of the source |30. Altercondenseri'liSS lisbonnec'ted @between the 'common terminal of 'the resistors |66 fand f 61 fand ground. 'The tube V5'lis Voonducting current at Iasubstantially constant irate pr'fior Lto the appearance of a negative pulse ion its-'grid' (duetothe 'oscillation of nthe tunedvcir- -cuit'Tn "but this pulse `is lof sufficiente magnitude ftmeauselthe Stube V5 .tob'e cut off. The positive portion of the voltagebscillation setup tin .the tuned circuitTi '(see `6C), however, `causes .lihe tubefaga'in toV c'onductfand` the :plate voltage toigdto aLloWer value than. it had been'prior to ithe arrival 'of thefpulse on its grid, andftloisvolt- `agergradually.clnangeslzto substantially :the same Valuel'ithadfbefore' the appearancetof:thefplsein -the :tube N4', jnfFig. i60. 'Thepositive rectangular pulse portion of the waveshown yn Fig. iiiDfis-.usedias ethciirst ".gate pulse.

:The .anode .|60fnf ltheiube M5 nisyconnectedtoa .'crcuitrincludingthe smallcondenser |10, `.of the order 1 of 2.5 micromicrofarads, Afor .example, `and fthe `resistor .111., which acts to .dierentiate the `voltage wave :applied ithereto :to :form the two :voltage 4pulses :shown :in Rig. 6E. The .negative pulse i shown `inilig. 6E` then excites Athe circuit ycomprising tubes N11, V8 and VS exactly :asxthe :pulse it'shown in 6A) at the terminal M21 excited :the circuit :comprising tubes LVB, V4 fand Viirtorform :thefsecond gate pulse.

The tube iVJ .includes .an anode 180, avcath'ode 18:1.,iacontrol `:grid 162, a .screen grid |83 yand yn suppressor grid .184. .An anti-sing resistor .fisconn'ected between the k.control grid 5| 82.andthe condenser |10 while a similar anti-sing" ristor .385 is oonnectedllbetween thefscreen. grid :183. and the resistor :|29 Athe :other terminal :of which `connected to the positive iterminal oi .the .source |30, :The suppressor Igrid :|84 :and the cathode .i331 .tare .nonnecte'd .togroundwhile .the anode |80 connected 4through the .resistor `|81ga'ndir1- :ductance :member `|88 .(for high .frequency `egual- -ization the resistor '|20 :to :the zpositive '.ter Aminal of thesourcezlll.

. .The :anode .1801s 'connected .through `the Y.cou- .pling condemer |89 :tothe :control fgrid |02 ,of .the `tubev which;alsofincludes an anode 90.2.@.cath- :ode ;|=,9 laxscreensgrid |83 and a suppressor grid 504. vThe :controligrid-.voltage of'theztube V8;is shown in Fig. fSE. yAnanti-.sng 'resistor i |95 if is -alsofconnected 'in the :lead tothe 'control grid .i 02ven'd arleakvresistor A|196 .is connected fbetween :fthe :coupling condenser A82 and ground. The .anode |90 .iszconnected .tl'irougb :the resistor ,|01 `and-.theresistor |248 tothefpositivefterminal ofthe 1,. Esource y|30 whilethe. screengrid n|93 isaconnected through fthe .antifsingfresistor :198, andthe re- .sistorz |48to 4the sameterminal of `:the source |30.

`.The suppressor grid |94 and thexcathode`f|9| :are :connectedthroughthe .parallelfconnected resistance .member l-45fand.capacity member "|46 yto ground, `these last itwo members providing .bias .for ythecontrolggrid 192. The bleeder Aresistor |41 is. also `utilized asthe-cat-hode of the tube V8 isconnectedftopoint -G :which is connected Vtc the :cathode of thel -tube NT4. It will befapparent from 4the drawingthatmany of the resistor land capacity members of.thefgatefforming circuit.No.;1 are utilized also in the gate-forming` :circuit .'No..21'in orderto save parts. f

.The .anode 00 f `is connected through the cou pling condenser 4.99 to @the interstage tuned v.cir-

cuit {Iz andalsotothecontrohgrid 2020i-the tube :Vflrfwhich also rincludes an anode .200.;a cathode 20| a screen grid 203,and-a suppressor grid i204.

.The tunedfcircuit T2V comprises .sa r.capacity i mem- `ber 205rand aniinductance memberi206'connected ...in `.parallel ybetweerrthe control grid V202 :and mground. ,Thefaction .of-.the tuned circuit 'Izis --similar tothatof the 'tuned circuit Ti in the gate- .forming `circuit No. .-1.

.The-ecathode. 20| f Aand the suppressongriditare connected to ground AWhile 1the A'screen fgrid -203 is k.connected through `an anti-sing resistor201.and theA resistance--mem -bers- |66 and. |61 to `thepositive terminal of the source |30 while the v1a-node .s200 4is connected V except that it is displaced in time by aperiod substantially equal to the duration of the positive pulse in the wave shown in Fig. 6D. The positive pulse of the wave shown in Fig. 6H forms gate pulse No. 2. Gates l and'2 are eectively rectangular since only the positive part of each of the waves shown in Figs. 6D and 6H is utilized.

The function of diodes V5 and Vlll is to limit the height of the positive portion of the gate pulses to a fixed Value. The plate 2|@ of the tube V6 is connected to the plate |99 of thetube V5 through a coupling condenser 2II while the cathode 2I2 is connected to the cathodes 21| and 29| of tubes VI3 and VI 4 to be hereinafter described. The plate 2|9 is connected through the resistance '2I5, the resistance 2I3, the top half of the resistance ZIE and the movable tap 2I1 to ground. Similarly, the anode 229 of the diode Vlil is connected through a coupling condenser 2| I to the plate 298 of the tube V9 while the cathode 222 is also connected to the cathodes 2'!! and 29|. The plate 229 is connected through the resistor 225, the resistor 2I4, the bottom portion of the resistor '253, and the movable tap'vZIl to ground. When the gating pulse No. 1 causes the plate 2I0 to exceed the potential of the cathodes of VI3 and VI4 the` tube V6 conducts and the top of the gating pulse is limited. A similark action takes place with respect to gating pulse No. 2in tube VIIl. The two gating pulses are then 'applied to integrating circuits 26 and 21 through terminals 226 and 221. The gate'pulses as they appear in the grid circuits of tubes VI3 and VI4 can be made to have the same amplitude by varying the position of the movable tap 2I1, which varies the potentiometer action of the associated resistors.

Also applied to the terminals A.225 and 221 are the output voltages of the'ampliers 28 and 29. The amplifiers 28 and '29 have applied thereto, in parallel, the video signal from the receiver I8, either directly or through the xed` delay circuit 43, the purpose of which is to compensate for the minimum delay of the variable delay line or unit 23 plus the delay represented by the width of one gate pulse if it is desired to Atrack down to very low ranges. f Y

The amplifier 2B preferably comprises a tube Vl I including an anode 239, acathode 23H, a control grid 232, a screen grid 233, and a suppressor grid 234. The control grid is connected through an anti-sing resistor 235 to an inner terminal 236 of the leak resistor 231 connected between the coupling condenser 238 and ground. The suppressor grid 234 and the cathode 23| are connected to ground while the screen grid 233 is'connected through an anti-sing resistor 239and a resistor 249 to the positive terminal of the source |39. The by-pass condenser 24| is connected betweenthe terminal of the resistor 239 remote from the screen grid 233 and ground. The anode 239 is connected through resistors 242 and 243 to the positive terminalof the source |39, a lter condenser 244 being connected between the common terminal of the resistors 242 and 243 and ground. The anode is connected through coupling condenser 245 to the terminal 225.

Tube VI2 includes an anode 250, a cathode 25|, a control grid 252, a screen grid253, and a suppressorgrid 254. The control grid 252 is con#- nected to an inner terminal 256 of the grid leak resistor 251 through an anti-sing resistor 255. The screen grid 253 is connected through an anti-sing resistor 258 and the resistor 240 to the positivexterminalrof the source |30, while the anode 250 is connected through the resistor 259 and the resistor 243 tothe same terminal. The cathode 25| andthe suppressor grid 254 are connected to ground. By means of the coupling condenser 260 the anode '250 is ccnnected'to'the terminal 221. r -v The amplified video signals from the ampliers 28 and 29 are applied tothe control vgrids Aofthe tubes`V|3 and VI4, along withthe 'gate pulses produced by the gate-forming circuits 24 and 25,;respectively; through anti-singv resistors 26| `and.262,frespectively. VI3 includes an anode 210, acathode 21|, a control grid 212, a screen grid 213', and'a'suppressor grid 214.- The ksuppressor grid 214 is connected to the cathode and through parallel-connected resistance 'member 215 and capacitymember 216 `to ground which combination provides bias for the control:g'rid. A bleeder resistor Y211 is provided between the cathode 21| and the `resistor' 218' which is con-` nected to the1p0sit1ve terminal `of the power supply 280 for direct current (which may be replaced bythe power supply 38 or |30, if desired).

The `plate`-21ll is connectedthrough resistor 281 to the voltage divider, consisting ofthe resistors 282 andj283.- Anantising resistor :i285 Vis connected between the screen grid`213 "andthe resistor 218, the common terminal of the resistors 285 and`218 being connected through a by-pass condenser 284 to ground; C onnected tothe plate 210 is an integrating"circuithaving a compara; tively long time constant, the integrating circuit comprising a condenser 286, of, for example, '.01 microfarad connected between the plate 210 and ground and the-resistor-28| of, Aior examplefl' .megohmV A resistor 281 i of, for example, 1 megohm, is connected between the plate210 and the control gridV 302 -of the tube VI5 andarcon denser 288 is connected between the control grid 302 and ground, this latter-condenser being of the order of .005 microfarad. The resistor 281V and the ondenser 288 filter out khigh frequency varia- The tube VI4 includes an anode 290, a cathod 29|, a control grid 292, a screengrid 293 and a suppressor grid .294.4 The suppressor grid-and the cathode are connected together and toi the cathode 21|. of the tube VI3,-thus making use of the resistor 215 and the condenser 216 topro- Vide bias -for the control grid 292; Thescreen grid 293 is connected-through an anti-sing're sistor 295 and resistors 218 and 219 to the positive terminal of the source 28|).l A lter condenser 289 is connected between the common terminal yofthe resistors 218 and 219 andground The anode 290 -is connected through resistance 296 to the lcommon terminal of the resistors282 and 283. The anode 290 is also connected to the integrating capacity member 291 similar to the capacity member 288. A resistance member 298 similar to the resistance member 281, and a capacity member 299 similar to capacity. member`28 are also provided to lter out high frequency var' iations. The resistance member 298 is connected to thev control grid 3I2 of the double triode VI5.v

Tubes V|3 and VI4 'are biasedso far beyond cut-off, in the absence of the gating pulses, that even the signal'overload peaks (positive) inthe video signal are inadequate t-o cause them to pass any plate current. However, whengate No. 1 is applied to the control grid 212, tube VI3 is made conducting to an amount` depending on the Video signal then present. Similarly, when gate No. 2 is applied to the control grid of tube VI4 it is `l.l made conducting to an amountl .depending on the video signal then present; rll'he integrating condensers 288 and 2951i in` the output: circuits ot the tubes;` V13 and VM.. respectively; are charged up:` negatively as these tubes become conducting during each., pulsing intervalV and retain the charges for several cycles. The voltages across thecondensers. 286' andf 291-r are the4 result of.' integratiorisA of the signals-A occurring during the corresponding gate: intervals. over the past. seuerol cycles. LNoise presentinthe signal, i because ol'its random nature, tendsP to integrateequally between. the two gate-controlled circuits. If an echoepuise from the object or target it isdesired to; track iscentered,l that. is, if it lies` half withinA the4 time span ofgatepulse No. 1 and halt within the time spani ofgate pulse No. 2; it also wm integrate, equally into the two` channels, but it the echo pulse drifts off into the timevspanof one or the other ofir the two gates, the integrated chargeY on. the integrating condenser in the plate circuit of the tube VM will increase or-decrease with. respect; to that on the one inthe platecircuitof the tube Vl3. A voltage unbal'ance will. thenbedeveloped between the grids 30% and 312 ofthe double triodelfl' which comprises one of the two tubes. of? a direct current amplierf 44.

The tube-V151 includes: an anode'llilll, a cathode 3M', and a; oontrolgrd .3!!2 fonone section an anode 31.0; a. cathode 3H, andl a control .grid M2 for' the second section. Ihe two cathodes 3M andl 3H are connected'. by a potentiometer 383, the. variabletapSM of' whichV is connected through. aresi'storSUS. to ground. The variable tap. 3M makes it possible tom-cate an unbalance uolta'gebetween thecathodes' of`V|5 which can be used for movingVl the` gate pulses" with \r'efer ence tothe corresponding transmitted pulsesV for "scanning* purposes,V aswill be pointed" out more fully below; Anode 38e-isconnected the resistors 3DG and 2PK-'9 to the positive terminal of the source 280 while anode Bill is uninfected. through resistors 3M and 2id to the same terminal of the power supply 2 89. AThe anode 3M is connectedthrougha voltage dropF pilflg4 resistor Ill't'o the control' grid4 322' of the double triodetube Vl 6= serving as aA second stage of the direct current ampli-iler; 44;` while the anode tllfoi the tube VI- isy connected through a.. voltage dropping resistor 399 to the control grid 332 of' the right-hand section ofV the.. tube Vt. The tube V16- also' includesH an anode 32.0' and?r a cathode 32E; and an'-ztnodev 331iL anda cathode- 33t; A grid leak` resistor 323 isV connected between the grid 322l andf groundA whilea similar resistor 333 isf connected between, the grid 332.-' and ground. The cathodes 32| and.33il are connectedto ground by means of similar resistors 324 and 3&5.'V The modes-323 and 330 are connected through the resistor 32E tothe positive terminali ofi the source 280; The cathodesj 321| and; 331 are connected to: abalanced; modulator' 43l the.. parallel 'connected resistance member 32lY and. curmcitymemberl 328fbeing: connected in one' of the leads torthe modulator. It will be seen thatV they` entire systerm, including` the.` motor 4'8?, the gear train between'the motor: anclswitch No. 1.0L the variable delay 'unit 23the, unit 23, the gate-forming circuits M- and` 2.5,jthe integrator tubesand circuits 25 and 21:-, the direct current amplifier 4 4, modulator 45;;and theL alternating current amplifier 1&6` follows it,.comprises; one large feedback loop structure. forming. what is commonly known as, a.servomechanism. The resistor SZTandcapacity 328. are included to .pros

12 videthe frequency 'characteristic'required to stabilize this loop.

The modulator: 45 comprises a bridge structure of four rectier elements 349', 341i, 342 and 34.3. An alternating current from a suitable source 4f?, which preferably is of the same frequency ,and phase as. that'of the oscillator I2, is applied through a Sil-degree phase shifting networl; 33,5 comprising theY resistors 345 and 346 ih series with the line and the condenser 34111 across. it, to the primary winding 348 of a transformer 343', the secondary winding ofA which is` connected to one diagonai of the. bridge, the other diagonal being connected to the primary winding 35i. of a transformer 352 the secondary winding 353 of which is connected toA a push-pull amplier 4'5. The leads from the cathodes of the tube Vl are connected: tothe mid-points ofthe windings 35!! and 35i, respectively.. The modulator 45 operates inaccordance with the description in Patent 2,025,158fissued December 24, 1935, to F. A. Cowan to, suppressfthe carrier from the. source and transmitto-the. output circuit substantially only the` upper andlower side-bands produced' byV the amplitude modulation of. this carrier byV the signal input.` The output wave of the modulator. is shown in Fig. 7B andwill be explained more fully below. This output wave from the modulator is applied to the input off a conventionaLpush-pull amplier 4'5, having the required amplification and a suicient output power-at the proper impedance level to drive onelwnding itil of anyV suitable .two-phase low inertiamotor fit. An. unmodulated 40u-cycle voltage is' applied to thewinding. 4!!! of the motor 4f; by means of a transformer 402, the terminals of the primary winding'llLB of which are connectedfto` the source 41. Since the carrier input to the modulator 45 isshifted degrees by the phase shifter 335, the output'` of the amplifier 4'6 will bear a plus o1.' minus 9U-degree phase relation to the fixed phase excitation of the motor depending on the direction of the direct current unbalance which drives the rn.odulator,. Any unbalance voltage resultingV from the; received signal" not occurring symmetrically with respect to the twogating pulses thereby causes rotation of the motor armature 404 which is mechanically connected to the variable delay unit4 23' to drive switch No. 1 thereof in. one direction or the other; The rotation is in az direction to center the gates about the rcvceiyed signal, reducing the unbalance of the drivingA voltage to zero. A dial, calibrated in thousands of yards of range, for example, indicates thedelay introduced by the unit. 23 and is an accurateV indication of'V the range. If desired, the diall may be connected directly to the motor 48, as: indicated by the reference character 5ml in Fig.. 1'.

A mechanical output unit 49 is shown. mechan` icallyconnected to the motor 48. This unit comprises a low inertia motor similar to the motor .48 and means for causing this motor to follow the; motor 48. The motor inthe output unit can be;used' to operate an opticalrange iinder or to provide a componentv of motion toa computer. A suitable follow-up system is shown in Patent 2,056,348 issued October 6, 1936to, M. A. Edwards.

The-operation of:` the system shown in Fig. 1 willnowbe described, reference beingV made also tothe circuit diagrams of Figs. 2, 3 and 4 and to the diagrammatic and graphical representations of Figs5,.6 and;'7. A` pulsed ultra-high frequency radio waveis; directed? atr an object by means of the` antenna.A H.. Thereilected wave isreceived 13 by antenna I1, amplie'd andfrectiiiedrby thereceiver I8 and sent to the fixed delay circuit43 in which it is delayed by an amount correspond'- ing to the minimum delay ofthe variable delay line 23 plus the time span of one of the gate rpulses (formed by circuit 24). This delay line, as pointed out above, may be omitted, if not .desired. The high frequency pulse (shown inA Fig. A) comprises, for example, a Wave of a radio frequency of 3300, megacycles per second, the duration of a pulse being approximately onefourth of a-microsecond and the pulses being -repeated four hundred times per second.. The an'- tennae by which this pulse is radiated and received are preferably of a. directional type and The video signal from the amplifier in.the're= ceiver I8 is passed either directly or throughl the xed delay circuit 43 into the two amplifiers '23 and 29 connected in parallel.

At this point it is desirable to leave the received signal temporarily and describe the formation of timing pulses which have the same periodicity as the transmitted pulses but which have a variable time relationship with respect thereto, as will be clear from the decription-below. VThe timing pulses are formed 'at the output of the variable delay line 23 and are used to initiate the forma tion of the gating pulses in the gating circuits 1 and 2. Fig. 5A represents a pulse modulated ultra-high frequency Wave applied to the antenna I I While Fig. 5B shows a corresponding reference-time pulse which can be generated in the pulse generator I3, for example, andwhich is fed to the delay line or unit 23 to form a timing pulse at a controllable period of time thereafter, Fig, 5C shows a timing pulse 4H.) produced at the output of the variable delay line or unit 23.' It will be understood that the pulse 4H)` may occupy positions to the right or left of that of the pulse shown in Fig. 5C, its position. being dependent on the delay of the-unit 23. Pulse 4H shown by dotted lines represents the output pulse as generated at the minimum delay setting of the delay device. Fig. 5D shows a received video signal as it might be seen on a cathode ray oscilloscope,

the significant portions of several pulsing cycles shown superimposed. The transmitted pulse M2 is shown yremoved from the reference line 453 by a distance corresponding to the xed delay introduced by the circuit 43. Echo pulses 4M and SI5 are also shown. In this dravving'the echoes shown are fairly Weak ones and the re ceiver gain is assumed to be turned up so high that the signal produced by thermal agitation in the input circuits and commonly called thermal noise is visible as a random Waviness in the oscilloscope trace. The delayed pulse or timing pulse produced by the variable delay lin'e 23 is amplied by the tube V3 and applied to the normally cut-off tube V4. The tuned interstage circuit T1 following the tube V4 is shock-excited and describes an oscillation as shown in Fig. 6C. Apparatus for and the method of forming osciln lation-s of the type shown in Fig. 6C are more completely disclosed in a copending application of B. M. Oliver, Serial No. 486,780, iiled May 13, 1943, and which issued as Patent No. 2,433,863 on January 6, 1948. The dotted` line 42! in Fig. 6C represents the cut-ofi' bias level of the tube V5. It will be seen that V5 is cut otfior a period of' time corresponding to the time between the points 422 and 423 in Fig. 6C. V5, being lcut off during this interval, forms at its plate a rectanguiar positive gate pulse 424 (the upper part of Fig. GD) and a curved lower portion 425. The p ve rectangular portion of the Wave shown in l1 6D is designated gating pulse or gate No. 1 and is later used to unblank tube V13. The output pulse from thetube V5 is differentiated by the circuit comprising the elements I'Iii and I'II to form a Wave such as thatshown in Fig.'6E having a positive pulse 42S and a larger negative pulse 2?. The grid voltage of the tube V8 is shown in Fig. 6F and comprises .a positive pulse @2S which is similar to the pulse in Fig'. 6B except that it is displaced by a period of time'very nearly equal to the width of the positive rectanguiar portion of the gate pulse No. 1 shown in Fig. 5D. The negative pulse 429 has no eiect on the tube V8 as it is already cut on". The voltage applied to the grid of tube V9 is shown in Fig. 6G. This voltage causes'an output Wave in the tube V3 of the shape shown in Fig.V 6H which, it will be noted, is similar to the wave shown in Fig. 6D except that it is displaced in time by a period of time very nearly equal to thewidth of the positive pulse of the Wave shown in Fig. 6D. The positive rectangular pulse shown in Fig. 6H forms gate pulse No. 2. In point of time, gate pulse No. 2 is contiguous to gate pulse No. 1, that is, the trailing edge ofthe rectangular top of gate No. 1 is simultaneous with the leading edge of gate No. 2. The received signal is introduced to the ampliers yi728 and 23 in parallel and after amplification-therein, is applied to the integrating circuits 26 and 21, respectively. Tubes VI3 and VI4 which supply the integrating circuits are biased so lfar beyondcut-ofl normally (that is, when no gate pulse'is applied tothe grid) that even the highest signal peaks (positive) which can be obtained even under overload conditions are inadequate to cause them to passv any plate current. (It is indicated in Fig. 5E that the video overload level isset 'at' a value such that'even the maximum signal output is insuiicientrtomake the tube VI3 conducting.) When gate No. I is applied to tube Vl 3, however, the plate of the diode -VB is brought up to, but not appreciably above, the cathode potential of tube VI3 whose control grid 212 is accordingly elevated to (or aY little' above) cut-off as determined by the associated resistor potential dividers. The signal occurring during this interval thus causes plate current vto ow in the tube VI3.v The video signal of Fig.' Y5D (including transmitted pulse 4I2. echo pulsesr414 and 4 l 5 and random voltage variations due' to noise, etc.) and the first gate pulseshovvn in Fig. 6D are shown as a combined Wave in Fig. 5E. 1The` plateicurrentA in the output of tube VI3 is shown in Fig. 5G, -all portions of the Waveof Fig.'5E-be1ow the cut-01T line in that gure not being reproduced in the output signal of this tube. Fig-5F shows a Wave similar to that of Fig. 5E eX- cept that itrepresents the grid voltage of the -tube vm rather'than that of the' tube vla, whileFig. 5H shows the plate current of the tube VI4. yIt will be noted tliata portion 438 of the energy of theechopulseAl 5 4appears'in Fig. 5G and a portion 43| thereof appears in Fig. 5H. In the plate' circuits of (the tubes VI3 land VI4 are the long time constant'integrating' circuits which collect Whatever negativer charge is passed by the tubes VI3 and VI4, respectively, during the selected interval of each pulsing cycle 'and store it forsev-V l eral cycles. Naturally, beinga linear network all the charge contributions ever contributed are superposed, and each contribution may be thought of as dying away exponentially as time progresses, so that the state of charge of the condensers is at all times determined by the few most recent cycles. Noise present in the signal, because4 of its random nature, tends to integrate equally into the two gates. If the echo pulse being tracked, that is, for example, the pulse 4|5 of Fig. 5D which represents a selected target, is centered, that is, if it occurs half within the time Span of gate No. l and half within the time span of gate No. 2, as indicated in Figs. 5G and 5H, the charges integrated into the two channels will be (on the average) equal. If, however, the pulse drifts, for example into the time span of gateNo. 2, the integrated (negative) charge on the integrating condenser in the plate circuit of the tube V|4 will increase with respect to that on the condenser in the plate circuit of the tube V|3;. An unbalance voltage will then be applied to the grids of the direct current amplifier VI 5, the grid 302 going positive with respect to the grid 3|2. The apliied unbalance on the plates of the tube VI'5 is applied to the grids of the tube V|6 actineas a dual cathode. follower. The potential difference on thecathodes of the tube V|6 is used to develop a control current which unbalances the copper-oxide balanced modulator comprising the bridge of rectifier elements 340, 34|, 342 and 343. This bridge has applied across one diagonal a L10U-cycle alternating,v current obtained through the- S-degree phase shifter 335. The copperoxide modulator then applies,V a 10D-cycle signal to the amplifier whose output power is used to drive the motor 48 in the proper direction tonadjust the contacts on the delay line 23 so` as to,` re,- center in time the gate pulses on the selected echo pulse. It will be apparent that increasing the delay in the delay line 23 increases the, time interval between the transmitted pulse and the corresponding gating pulses and decreasing. the delay interval produced by the line 23 will have the opposite effect on the gating pulses position with respect to the transmitted pulse;` The variation with respect to` time of the, input signal voltage applied to the modulator is represented in Fig. 7A, whilev Fig. '7B represents the output of the amplier 4G. which is applied to the. motor winding 400. It will be noted that the. portions ofV the wave designated -90 degrees in Fig.V 7B are 180 degrees out of phase withthose. designated +90 degrees. (The plus or minus 90- degree relationship is with respectv to; the unmodulated wave of 40G-cycle, frequency applied to the-winding 40| ofthe motorv 48 from the alternating current source 4l.) When the phase` of the current in the winding 40| is +90 degrees with respect to that of the current in the winding- 400, the motor 48 is driven in onev direction while if itis -90 degrees with respect to the current in the winding 403, the motor is driven in the opposite; direction. Fig; 7C represents theV torque on the motor 48; corresponding to the input Wave shown.

The delay of the echopulse 4|5 in Fig. 5Dirom the transmitted pulse 4|2 is (when the echo is centered with respect to the two gate pulses). equal to the delay of the line 23 plus the duration of one gate (gate No. 1). Range to` the selected target can therefore be read from the setting of the delay line switch or the revolutions ofthe spinner-motor shaft.

Byunbalancing the. potentiometer in the cathode: circuit of the tube VIE, the device can be made to. scanin the absence of any echo, that is, move the two gating pulses in either direction in Search ofv an echo. However, for. manual drive or slewing a double push-button unit shown in Fig. 4 is provided (the push-buttons themselves have notv been shown in the drawing but one is arranged to actuate contact member 45| to the right and the other one is arranged to actuate contact member 454 to the left when the respective. buttons are depressed). When neither push-button is depressed the equipment will follow an., echo, or lacking one scan as indicated above until the target is picked up. Depressing one., button of the manual control equipment 450 closes the contacts 45| and 452 while depressing the'other button closes the contacts 453 and 454. This shorts to ground one or the other of the control grids 322 and 332 of the tube V|5 and the severe unbalancewhich this produces at the ampliier' input overpowers any signal unbalance which may be present, and drives the motor in the indicated direction at top speed, thereby movingthe gating pulses in a direction corresponding toa greater or a lesser range and at maximum speed. This is known as slewing The manual drive control keys oi the equipment 45B must be accessible to the pilot or gunner sighting the target. In addition to sighting the target, the pilot or gunner must then estimate the distance to the4 target, slew the equipment past undesired targets into a range in the vicinity of the range of the desired target and then allow the equipment to scan, preferably at a constant rate of speed, to the desired target whereupon it will automatically follow the target until pulled off by the manual drive control 45S. If the. equipment is adjusted to scan in, it must bev slewed to a. range greater than the target, or conversely, if adjusted to scan out, it must be slewed to a range less than the target. Visibility to the operator of a range dial is a great convenience in this type of operation.

Itis obvious that the variable delay means of this invention can have many other uses other than in connection with a radar system. It can be used fordelaying signals or pulses in telephony or in television or in pulse position modulation systems, for example.

Various changes may be made in the embodiment described above without departing from the spirit4 of the invention as indicated by the claims.

What is claimedA is:

1. A variable delay transmission line for pulses comprising input and output terminals, a group of similar signall delay sections connectedV in tandem, a switch member movable in two directionsY and connected to` one of the terminals of saidtransmission line, a group of contact members positioned' to be contacted one at a time by said switch member, means for connecting an input terminal of said tandem arrangement to one of said contact members and for connecting an output terminal of each section to others of said contact members in order, a second group of signal delay sections each producing a signal delay which is. substantially equivalent to that of one o f said first group of delay sections multiplied by the number of sections in the rst group. the sections of said second group being connected in tandem with an input terminal of said second tandem arrangement .being connected to an outputfterminal' of thetandem arrangement of said 17 rst group of delay sections, a second switc member movable in two directions and connected to the other terminal of said transmission line, a second group of contact members positioned to be contacted one at a time by said second switch members, means for connecting an input terminal of said second tandem arrangement to one of the contact members of said second group Vand for connecting an output terminal of each section in said second group to others of the contact members of said second group in order, and means including counter-gear mechanism for causing the second movable switch member to snap from one contact member in the second group to 'an adjacent contact member in that group in one direction when the rst movable switch member snaps from the first to the last contact in the first group and for causing the second switch member to snap from one contact member in the second group to an adjacent contact member in that group in the opposite direction when the first movable Aswitch memberV snaps from the last to the rst contact member in the rst group.

2. The combination of elements as in claim 1 in which said two movable switch members are driven by a reversible motor.

3. The combination of elements as in claim 1 in which each of said delay sections comprises a series inductance member and a shunt capacity member.

4. The combination of elements as in claim 1 in which each of said delay sections comprises a series inductance member and a shunt capacity member, all of the inductance members in a group of sections being wound on a common form in series aiding direction thereby providing mutual inductance between the inductance members of adjacent sections.

5. The combination of elements as in claim 1 in further combination with a plurality of voltage dividing means connected respectively at the terminations of the sections of said second group, the voltage dividing means serving to attenuate the output pulse of the section connected to the terminal of the transmission line through said secondmovable switch member, the attenuation being progressively less as the number of sections connected between the terminals of said transmission line increases.

BERNARD M. OLIVER.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,435,009 Kellogg et a1 Nov. 7, 1922 2,183,725 Seeley Dec. 19, 1939 2,263,376 Blumlein et al Nov. 18, 1941 K FOREIGN PATENTS Number Country Date 487,338 Great Britain June 20, 1938 

